As more than 50% of the world polymer production consists of polyolefins, increasing the molecular precision of polyolefin synthesis has the potential to directly improve the physical and mechanical properties of most of the objects surrounding us. Telechelic and semi-telechelic polymers, polymers with one and two functional end-groups, can be used as building blocks for the synthesis of such advanced polymers (FIG. 1). Telechelic polymers are most commonly synthesized by living radical or living anionic polymerizations. These polymerization methods, however, are not compatible with simple olefins and therefore telechelic polyolefins require alternative strategies to access directly.
Different approaches have been reported for the synthesis of telechelic polyolefins. The selective quenching of the anionic polymerization of butadiene with a capping agent (e.g. oxygen, carbon dioxide or epoxide) followed by the hydrogenation of the polymer to yield telechelic polyethylenes (Pes) remains the most common technique. The ring opening metathesis polymerization of cyclic olefins in presence of a chain transfer agent also results in telechelic polyolefins. While very elegant, these approaches do not employ the widely available ethylene and propylene monomers. Mecking et al. (Angew. Chemie Int. Ed. 2016, 55, 14378) recently reported the direct synthesis of telechelic polyethylene by copolymerization of 2-vinylfuran with ethylene. The unique reactivity of the catalyst toward the comonomer enables the exclusive formation of polyethylene with di-furan end-groups.
The living catalytic polymerization of olefin has been reported to yield semi-telechelic polyolefins. The main drawback of this approach, however, is that each molecule of catalyst yields only one polymer chain. Chain transfer polymerization (CTP) offers a viable solution to this issue. Indeed in a CTP, the use of a chain transfer agent (CTA) results in the formation of multiple polymer chains per catalyst, with the CTA being incorporated as one of the end groups. For example, lanthanide based catalysts have been reported to catalyze the chain transfer polymerizations of ethylene with electron rich CTA such as amines and phosphines to yield the corresponding amine and phosphine-terminated polyethylenes. Electron poor CTAs such as boranes, silanes and alanes have also been successfully incorporated into the terminus of polyethylenes.
Most of the examples of CTP involving olefins rely on oxophilic catalysts (early transition metal and rare earth metal complexes). The oxophilicity of these catalysts severely limits the scope of functional groups that one can introduce at the end of a polyolefin. In fact, thus far, linear semi-telechelic polyolefins produced by CTP have been reported to include at most one heteroatom per chain. Accordingly, there is a need for an alternative method to prepare end-functionalized polyolefins.